Pre-existing conditions cease to be insurance. From that point, you are talking about paying for expenses that can readily be anticipated. If insurers are required to offer coverage at premiums without regard to those conditions, the premiums for everyone will inevitably rise. As you query, people certainly would choose to not purchase insurance until they needed it, so further driving prices up — only sick people with near certain claims would opt for insurance.
Of course, such compulsory means exercised upon insurance providers would almost certainly be accompanied by compulsion upon insurance consumers. The anticipated method seems to be fines laid upon people who fail or refuse to purchase insurance. It is not clear how exactly those mandates will work upon the poor or what other exclusions or mandates could arise.
However these programs evolve, it is clear that they will continue to over-ride insurance mechanisms. Prices will reflect less the cost of insurable risks and will represent, rather, the costs of the socialized programs to spread health care expenses over society.
In my opinion, it is worthwhile to discuss means to provide for people who are less fortunate. Opposing socialist programs does not mean opposing their goals. I oppose them because they will hinder our ability to manage *actual* risks. Insurance prices provide vital signals to society that, when not hampered, can guide us to change behavior. Lacking those signals, it is more likely, in my view, that health care costs will skyrocket.
We could care for the poor more directly without destroying the insurance market.
EDIT: Reading back over my answer, I wanted to clarify something. “Destroying the insurance market” does not necessarily mean driving insurance companies out of business. Indeed, given their (sadly, “our”) powerful lobbying presence, that is highly unlikely. The destruction of the insurance market would mean that insurance companies in name would no longer be providing insurance but would rather be appendages to the social programs.